1. Field of the Invention
The present invention relates to a device for the guidance in rectilinear translation, in a given direction, of an object that is mobile in relation to a fixed object.
In general, systems of guidance with low amplitude of displacement use the properties of thin and elongated bands embedded at one end and subjected to forces at the other end. These bands have the property of lending themselves easily to flexing motions in the direction perpendicular to the band while at the same time remaining rigid in the parallel directions.
2. Description of the Prior Art
To obtain a motion of guidance in translation from the flexional properties of thin bands, it is possible to make a device consisting of a parallelogram comprising two equal and parallel flexible bands that are embedded at one end and connected at each of their other ends to a rigid element. Under the effect of an external force dictating a flexing motion to the thin bands, the rigid element shifts while remaining parallel to itself. Thus guidance in translation is obtained for a mobile object joined to said rigid element in relation to a fixed object that is joined to the embedding point of the bands.
When the strains are calculated so that they remain well within the elastic limits and the buckling loads, a perfectly precise and reproducible device is obtained, free from play, friction or wear and tear, and with no phenomena of jamming. The reproducibility is limited only by the residual hysteresis applied to the non-compensated effects due to the variation of the loads and the finite rigidity of the bands. To these advantages, we must add the possibility of making complex systems in a small number of monolithic parts, thus helping to improve reliability and reduce the cost of assembly.
However, the simple parallelogram configuration described here above does not provide perfect guidance in rectilinear translation. Indeed, in addition to the rectilinear translation proper that is sought, there is an error component, perpendicular to the direction of the motion, due to the apparent shortening of the flexible bands which tends to make the rigid element of the parallelogram approach the point of embedding.
To overcome this drawback, there is a known way of improving the rectilinear character of the motion of translation of the mobile object by introducing a second guidance parallelogram in accordance with FIG. 1 which is a drawing of a prior art device for guidance in rectilinear translation.
Briefly, the guidance device of FIG. 1 has a first intermediate parallelogram 10 constituted by two intermediate bands 11 that are flexible in the direction T of translation, fixed at one end 111 to a fixed base 1, joined to said fixed object, and fixed at another end 112 to an intermediate rigid element 12 parallel to the direction T of translation. The second parallelogram 20 is the guidance parallelogram series-connected inside the first parallelogram but opposite so that the two errors of transversal displacement compensate for each other. More specifically, the guidance parallelogram 20 has two guidance bands 21 that are flexible in the direction T of translation, fixed at one end 211 to the intermediate rigid element 12 and at another end 212 to a rigid guidance element 22 that is also parallel to the direction T.
However, although it can be used to a great extent in order to compensate for the errors of lateral displacement of the simple parallelogram, the prior art guidance device shown in FIG. 1 still has two drawbacks:
the precision of compensated guidance is highly sensitive to the longitudinal position of the intermediate rigid element 12 whose translation, when the flexible bands 11 and 21 of the parallelograms 10 and 20 are of equal length, must be exactly equal to half the translation of the rigid guidance element 22; PA1 the intermediate rigid element 12, which is attached solely to the bands 11 and 21 whose rigidity in the direction T of translation is low, has a mode of resonance at low frequency, excitable in the motion of translation, that limits the dynamic performance characteristics of the device. PA1 a first parallelogram, called an intermediate parallelogram, comprising firstly a pair of bands that are flexible in the direction of translation, known as intermediate bands, substantially equal and parallel to each other, each of said intermediate bands being embedded at a first end in a fixed base, joined to said fixed object and, secondly, an intermediate rigid element, parallel to the direction of translation, fixed at each of its ends to a second end of the intermediate bands; PA1 a second parallelogram, called a guidance parallelogram, opposite the first parallelogram and comprising, firstly, a pair of bands that are flexible in the direction of translation, known as guidance bands, substantially equal and parallel to each other and to the intermediate bands, each of said guidance bands being fixed, at a first end, to said intermediate rigid element and, secondly, a rigid guidance element, parallel to the direction of translation, joined to said mobile object and fixed at each of its ends to a second end of the guidance bands, the guidance device being compatible with a reduction by half of the motion of the intermediate rigid element in relation to the motion of the rigid guidance element, in a manner that is simple, efficient, without play and without friction. PA1 at a first end to said fixed base by means of a first flexible reduction band; PA1 at a second end to the rigid guidance element by means of a second flexible reduction band; PA1 in its middle to the intermediate rigid element by means of a third flexible reduction band.
In order to resolve these difficulties, it has been proposed to link the two rigid translation elements 12 and 22 to an external motion reduction device in which the intermediate element 12 is moved in translation by a gear wheel that is engaged with a gear wheel having twice the radius activating the translation motion of the guidance element 22. Thus, the ratio of two that must exist between the respective displacements of the intermediate and guidance elements is ensured.
However, a known device of this kind can hardly be made in practice because of its complexity and space requirement as well as the non-linear stresses of friction and play introduced by the reduction gear.